This invention relates to an O-ring seal for retaining the lubricant around the journal bearings in a rock bit or drill bit for drilling oil wells or the like.
Heavy-duty drill bits or rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam, and the like. Such drill bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled. As such a rock bit is used for drilling in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a drill bit in such severe environments is in the order of 20 to 200 hours for bits in sizes of about 61/2 to 121/4 inch diameter at depths of about 5000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical.
When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation. Prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or severe wear of the journal bearings on which the cutter cones are mounted. These bearings are subject to high pressure drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. Considerable development work has been conducted over the years to produce bearing structures and to employ materials that minimize wear and failure of such bearings.
The journal bearings are lubricated with grease adapted to such severe conditions. Such lubricants are a critical element in the life of a rock bit. A successful grease should have a useful life longer than other elements of the bit so that premature failures of bearings do not unduly limit drilling. Failure of lubrication can be detected by generation of elevated pressure in the bit, evidence of which can often be found upon examination of a used bit. The high pressure is generated due to decomposition of oil in the grease, with consequent generation of gas when lubrication is deficient and a bearing overheats due to friction. Lubrication failure can be attributed to misfit of bearings or O-ring seal failure, as well as problems with a grease.
Pressure and temperature conditions in a drill bit can vary with the time as the drill bit is used. For example, when a "joint" of pipe is added to the drill string, weight on the bit can be relieved and slight flexing can occur. Such variations can result in "pumping" of the grease through O-ring seals, leading to loss of grease or introduction of foreign materials, such as drilling mud, that can damage bearing surfaces.
One of the consistent problems in drill bits is the inconsistency of lifetime. Sometimes bits last for long periods, whereas bits which are apparently identical operated under similar conditions may fail with a short lifetime. One cause of erratic lifetime is failure of the bearings. Bearing failure can often be traced to failure of the seal that retains lubricant in the bearing. Lubricant may be lost if the seal fails, or abrasive particles of rock may work their way into the bearing surfaces, causing excessive wear.
Rock bit O-rings are being called on to perform service in environments which are extremely harsh. Modern bits are being run at exceptionally high surface speeds, sometimes more than 500 feet per minute. One face of the O-ring is exposed to abrasive drilling mud. The life of the O-ring may be significantly degraded by high temperatures due to friction (as well as elevated temperature in the well bore) and abrasion.
It is therefore desirable to provide a consistently reliable O-ring seal for maintaining the lubricant within rock bits, that has a long useful life, is resistant to crude gasoline and other chemical compositions found within oil wells, has high heat resistance, is highly resistant to abrasion, has a low coefficient of friction against the adjacent seal surfaces to minimize heating, and that will not readily deform under load and allow leakage of the grease from within the bit or drilling mud into the bit.